Piston or rod seals are often subject to a design requirement that materially reduces life expectancy of the seal, namely, that the seal reciprocate over a port that permits radial extrusion of the seal element into the port with resultant tearing or abrasion of the seal. This problem has heretofore been solved by fabricating the seal element from relatively hard, non-extrudable material. However, since such materials are relatively inelastic, such known seals must be split to accommodate the radial expansion necessary for assembly on the piston. Obviously, splitting of the seal element induces leakage thereof.